Suspension polymerized aqueous absorbent polymer particles

ABSTRACT

A process for making clusters of individual polymer beads bonded together to minimize attritition in use or processing but characterized by an open structure for fast absorption of aqueous solutions comprises suspending a monomer mixture in a continuous phase at high shear agitation such that fine droplets of monomer are formed in an inert organic phase. A suspending agent which prevents the clusters from coalescing is provided. A first polyethylenically unsaturated crosslinking monomer, having substantial solubility in the aqueous phase, and an organic phase soluble initiator system are provided such that polymerization at the surface of the polymer particles occurs to form the porous clusters of the invention, A second substantially oil soluble crosslinker is optionally provided to further crosslink the clusters such that they do not coalesce.

EXAMPLES 1-29

An aqueous monomer mixture is formed by adding to 172.8 grams of water: 116 grams of acrylic acid; 110 grams of a 50 percent aqueous solution of sodium hydroxide; 0.23 grams methylenebisacrylamide; 4.6 grams of a 10 percent aqueous solution of VERSENEX 80 chelating agent (a pentasodium salt of diethylenetriamine pentacetic acid for scavenging metal compounds) supplied by The Dow Chemical Company; and, as a first portion of a partially continuous organic phase soluble crosslinker, the amount of trimethylolpropane triacrylate (TMPTA) indicated in Table I as "first crosslinker".

A continuous medium organic phase is formulated at low speed in a Waring blender/mixer, including 400 grams of ISOPAR™ M, a hydrocarbon supplied by Exxon Chemicla Co; and a suspending agent that is a mixture of 0.3 grams of AEROSIL™ R-972 (a hydrophobic silica supplied by Degussa, Inc; and 0.48 grams of a copolymer of 1 percent by weight acrylic acid and 99 percent by weight laurylmethacrytate.

To 200 g of the aqueous monomer solution, 0.5 mL of a 6.1 percent aqueous solution of sodium persulfate is added. The organic phase is mixed for 10 seconds at low speed in the Waring blender. The speed of the blender is increased to 800 rpm and the 200 g of the aqueous solution is added and stirred for 30 seconds. The resulting suspension is transferred to a reactor, where it is purged for 30 minutes with nitrogen. To the purged mixture, 1 mL of an 8.9 percent aqueous solution of t-butyl hydroperoxide is added. The suspension is purged an additional 10 minutes.

The polyacrylate polymerization is initiated by bubbling sulfur dioxide diluted in nitrogen into the reaction mixture. The agitation rate in the reactor is as indicated in Table I.

When the mixture temperature reaches 50° C., (unless otherwise indicated in Table I), the indicated amount of trimethylolpropane triacrylate as a "second crosslinker" is added. The reaction proceeds exothermically and is maintained at a temperature of 65° C. for one hour.

The ISOPAR hydrocarbon organic phase is then removed by filtration and the polymer product dried. The dry polymer is slurried in a mixture of 0.03 grams of VORANOL™ 2070 polyol, available from The Dow Chemical Company (Midland, Mich.), in methanol to provide 1 weight percent polyol based on the weight of the polymer material. The methanol is removed by vacuum stripping at 80° C.

The aqueous absorbent polymer material of the invention which comprises clusters of a plurality of individual particles of absorbent polymer, is analyzed for particle size distribution, vortex rate, 5 minute 0.3 psi AUL and 60 minute 0.3 psi AUL. These data appear in Table 1 below.

                                      TABLE I     __________________________________________________________________________                 Amount of                 second                    Centrifuge          Amount of first                 crosslinker                         Agitation                              Vortex rate                                    AUL (g/g)                                           Capacity     Example          crosslinker (g)                 (g).sup.1                         rate (rpm)                              (sec) 5 min                                       60 min                                           (g/g)     __________________________________________________________________________     1    0.345  0.55    400  41    16.9                                       20.0                                           19.1     2    0.345  0.55    600  2.6   23.0                                       24.2                                           22.9     3    0.345  0.55    800  15.7  22.6                                       23.6                                           20.4     4    0.345  0.55    800  5.0   24.3                                       25.1                                           22.6     5    0.46   0.55    400  3.9   25.1                                       26.0                                           24.0     6    0.46   0.55    600  6.0   24.8                                       25.4                                           23.2     7    0.46   0.55    600  15.4  20.7                                       22.6                                           19.3     8    0.46   0.55    600  3.1   24.9                                       25.8                                           24.3     9    0.46   0.55    600  2.9   24.8                                       26.4                                           24.3     10   0.46   0.55 (40° C.)                         600  2.3   23.7                                       24.6                                           22.5     11   0.46   0.55 (45° C.)                         600  5.3   23.1                                       24.4                                           22.4     12   0.46   0.55 (45° C.)                         600  5.4   23.3                                       24.7                                           22.8     13   0.46   0.55 (54.5° C.)                         600  2.3   23.7                                       24.6                                           22.5     14   0.46   0.55 (54.5° C.)                         600  5.5   23.1                                       24.5                                           22.2     15   0.46   0.55 (54.5° C.)                         600  8.6   25.1                                       25.6                                           24.2     16   0.46   0.55 (54.5° C.)                         600  2.8   15.0                                       15.3                                           13.8     17   0.46   0.55 (54.5° C.)                         600  8.9   25.1                                       26.9                                           24.8     18   0.46   0.55 (54.5° C.)                         600  10.3  23.7                                       24.0                                           21.9     19   0.46   0.55 (54.5° C.)                         600  9.4   24.8                                       26.5                                           23.3     20   0.46   0       600  9.8   21.0                                       22.6                                           20.8     21   0.46   0.55    800  5.3   25.5                                       26.2                                           24.0     22   0.46   0.55    800  4.5   24.2                                       25.4                                           23.0     23   0.46   0.55    800  5.6   23.1                                       25.1                                           24.8     24   0.46   1.65    400  39     6.0                                       19.7                                           22.4     25   0.46   1.65    600  14.5  21.4                                       23.3                                           19.8     26   0.46   1.65    800  14.9  21.0                                       23.0                                           21.3     27   0.575  0.55    400  4.0   23.9                                       25.9                                           24.5     28   0.575  0.55    600  2.7   23.4                                       24.8                                           24.0     29   0.575  0.55    800  2.2   24.6                                       25.9                                           24.4     __________________________________________________________________________      .sup.1 Second crosslinker was added when reaction mixture was at      50° C., unless otherwise indicated

EXAMPLES 30-43

The process of Examples 1-29 is repeated, except that 97 grams of the 50 percent aqueous solution of sodium hydroxide, rather than 110 grams is used and except that no methylenebisacrylamide is used. Further modifications are indicated in Table II, which provides the amount of trimethylolpropane triacrylate utilized as a first and second crosslinker and the amount of suspending aids used. The second crosslinker is added when the reaction mixture is at 50° C. The agitation rate during reaction is 500 rpm, except for Examples 31 and 38, in which it is 600 rpm.

Examples 30-43 indicate that systems which employ solely a predominantly oil phase soluble crosslinking agent are feasible.

                                      TABLE II     __________________________________________________________________________          Amount of suspending          aids Present                   Centrifuge          wt % based on AA                    Amt of TMPTA                            Vortex rate                                  AUL (g/g)                                         Capacity     Example          AEROSIL               LMA/AA                    added   (sec) 5 min                                     60 min                                         (g/g)     __________________________________________________________________________     30   0.776               0.414                    0.29                        0.29                            2.5   -- --  25     31   0.776               0.414                    0.29                        0.29                            4.1    4.4                                      7.4                                         26.5     32   1.034               0.828                    0.35                        0   8.4    4.5                                      8.3                                         27.8     33   1.034               0.414                    0.35                        0.58                            3.1   18.5                                     19.6                                         19.5     34   1.034               0.414                    0.35                        1.2 3.3   19.9                                     20.4                                         17.7     35   1.034               0.414                    0.35                        1.7 1.5   10.6                                     17.6                                         25     36   1.034               0.414                    0.35                        0.29                            1.8   18.5                                     22.7                                         22.6     37   1.034               0.414                    0.35                        0.35                            2.6   -- --  24     38   1.034               0.414                    0.35                        0.35                            9.1   -- --  22.2     39   0.517               0.414                    0.35                        0.35                            4.0   19.0                                     20.1                                         18.2     40   0.776               0.414                    0.35                        0.35                            4.7    5.2                                      8.5                                         23.3      41* 1.034               0.828                    0.46                        0   3.1   -- --  --     42   1.034               0.414                    0.46                        0   4.1   -- --  --     43   0.507               0.828                    0.92                        0   11.3  -- --  --     __________________________________________________________________________      *Rather than a 2:1 ratio between the inert organic phase and the aqueous      phase, a 1.5:1 ratio is employed.

Examples 44-69

The process of Examples 1-29 is repeated, except that in Examples 44-60 the amount of Aerosil R972 employed is 0.6 grams and the amount of the polylaurylmethacrylate/acrylic acid copolymer employed is 0.96 grams. In Examples 44-69, the amounts of trimethylolpropane triacrylate used as the first crosslinker and as the second crosslinker are 0.46 and 0.55 grams, respectively, the second crosslinker being added when the reaction mixture is at 50° C. The ratio of the weight of the inert organic phase to the weight of the aqueous monomer solution and the agitation rate during reaction are as indicated in Table III.

                                      TABLE III     __________________________________________________________________________          Oil:  Agitation rate                              0.3 psi AUL                                      Centrifuge          Aqueous                during reaction                       Vortex rate                              (g/g)   Capacity     Example          ratio (rpm)  (sec)  5 min                                  60 min                                      (g/g)     __________________________________________________________________________     44   2:1   600    6.0    24.8                                  25.4                                      23.2     45   2:1   600    15.4   20.7                                  22.6                                      19.3     46   2:1   600    3.1    24.9                                  25.8                                      24.3     47   2:1   600    2.9    24.8                                  26.4                                      24.3     48   1.75:1                600    4.1    24.9                                  26.0                                      23.4     49   1.5:1 600    4.0    26  26.3                                      23.3     50   1.25:1                600    4.7    24.6                                  26.5                                      23.5     51   1.0:1 600    27.1   19.4                                  20.8                                      20.0     52   1.75:1                500    5.2    24.6                                  24.9                                      21.8     53   1.5:1 500    7.0    24.2                                  24.4                                      24.1     54   1.25:1                500    --     --  --  --     55   1.0:1 500    --     --  --  --     56   2:1   400    3.9    25.1                                  26.0                                      24.0     57   1.75:1                400    --     --  --  --     58   1.5:1 400    --     --  --  --     59   1.25:1                400    --     --  --  --     60   1.0:1 400    --     --  --  --     61   2:1   600    14.9   22.7                                  23.3                                      20.7     62   1.5:1 600    38.9   20.2                                  21.3                                      19.2     63   1:1   600    11.2   21.0                                  24.2                                      21.2     64   2:1   500    --     --  --  --     65   1.5:1 500    7.3    24.6                                  25.0                                      21.8     66   1:1   500    8.2    23.9                                  24.5                                      21.2     67   2:1   400    11.4   22.9                                  23.7                                      21.3     68   1.5:1 400    9.6    23.5                                  24.3                                      22.0     69   1:1   400    --     --  --  --     __________________________________________________________________________

EXAMPLES 70-80

The process of Examples 1-29 is repeated, except that no methylenebisacrylamide is used, with diethyleneglycoldiacrylate being provided as a first crosslinker in the amount indicated in Table IV. In Examples 70-76, trimethylolpropane triacrylate is provided as an additional first crosslinker; in Examples 77-80, trimethylolpropane triacrylate is provided as a second crosslinker when the reaction is at a temperature of 50° C. The agitation rate during reaction and the amounts of trimethylolpropane triacrylate employed are as set forth in Table IV.

                                      TABLE IV     __________________________________________________________________________          Amt of DEGDA                  Amt of TMPTA        0.3 psi AUL                                              Centrifuge          (ppm based on                  (ppm based on                          Agitation                                Vortex rate                                      (g/g)   Capacity     Example          acrylic acid)                  acrylic acid)                          rate (rpm)                                (sec) 5 min                                          60 min                                              (g/g)     __________________________________________________________________________     70   4,000   4,000   500   3.2   18.9                                          23.9                                              25.2     71   4,000   8,000   500   2.1   21.7                                          25.5                                              25.2     72   6,000   6,000   500   4.0   24.1                                          25.1                                              25.1     73   8,000   4,000   500   5.7   24.1                                          24.5                                              22.4     74   8,000   6,000   500   4.3   27.0                                          27.3                                              26     75   10,000  4,000   500   4.1   24.8                                          26.1                                              25.3     76   10,000  6,000   500   5.2   21.1                                          22  21.7     77   3,000   10,000  400   9.6   14.9                                          15.1                                              15     78   4,000   10,000  500   4.8   18.4                                          19.8                                              19.1     79   6,000   5,000   500   14    8.9 15.4                                              17.3     80   6,000   10,000  500   5.0   21.1                                          21.7                                              20.7     __________________________________________________________________________ 

What is claimed is:
 1. A process for preparing a polymer material, consisting essentially of:(a) suspending an aqueous monomer mixture comprising a water soluble ethylenically unsaturated monomer in an inert organic liquid to form a suspension having a discontinuous phase of monomer droplets, said monomer mixture including a first polyethylenically unsaturated crosslinker having a solubility in water of at least about 0.1 grams crosslinker to 100 grams water, said suspension further containing a polymerization initiator which will partition such as to provide at least about 10 ppm polymerization initiator in said inert organic liquid, said suspension further containing a suspending agent to partially stabilize said suspension; b) initiating the polymerization of said monomer and said first crosslinker such that a crosslinking polymerization occurs at the interface between the monomer droplets and organic liquid, wherein individual particles of polymer are bonded together to form clusters and wherein during said polymerization, a second polyethylenically unsaturated crosslinker having a solubility in the organic liquid continuous phase greater than about 0.1 grams per 100 grams organic liquid continuous phase is added; and c) further polymerizing said clusters and second crosslinker such that further polymerization occurs at the cluster/organic phase interface.
 2. The process of claim 1 wherein said ethylenically unsaturated monomer is an amide, carboxylic acid or its esters, vinyl amine, or their salts or mixture thereof.
 3. The process of claim 1 wherein said polymer is crosslinked polyacrylic acid, crosslinked sodium polyacrylate or a crosslinked copolymer of acrylic acid and sodium acrylate.
 4. The process of claim 1 wherein said monomer mixture also includes a component which graft polymerizes onto one or more other monomer components of the mixture.
 5. The process of claim 4, wherein said component is polyvinyl alcohol.
 6. The process of claim 5, wherein said clusters are greater than 75 micrometers in diameter.
 7. The process of claim 6, wherein 2 grams of said clusters have a vortex rate of absorption of 50 grams of a 0.9 percent aqueous sodium chloride solution of less than 60 seconds.
 8. The process of claim 1 further comprising:(e) applying a wetting agent to said clusters.
 9. The process of claim 8, wherein said wetting agent is a polyol.
 10. The process of claim 1 wherein said suspending of said monomer mixture to form fine monomer droplets comprises agitating said monomer mixture at high shear rates.
 11. The process of claim 1 wherein said suspending agent comprises a cellulose ether, provided in an amount of 0.5 weight percent, based on the weight of said monomer mixture.
 12. The process of claim 1 wherein said suspending agent comprises a nonionic surface active composition having an HLB value greater than
 7. 13. The process of claim 1 wherein said suspending agent comprises a mixture of hydrophobic silica and a copolymer of lauryl methacrylate and acrylic acid.
 14. The process of claim 1 wherein said first crosslinker is trimethylol propane triacrylate (TMPTA), the diacrylate or dimethacrylate of ethylene glycol, the diacrylate or dimethacrylate of diethylene glycol, methylenebisacrylamide, triallyamine, tetraallyloxyethane, or combinations thereof.
 15. The process of claim 14, wherein said first crosslinker comprises TMPTA in an amount of at least about 2000 ppm to less than 8000 ppm, based on the weight of said monomer.
 16. The process of claim 14, wherein said second crosslinker is trimethylolpropane triacrylate (TMPTA) or tetraallyloxyethane.
 17. The process of claim 16, wherein the second crosslinker comprises TMPTA in an amount from about 5000 ppm to about 20,000 ppm, based on the weight of said monomer.
 18. The process of claim 1 wherein said polymerization initiator is t-butyl hydroperoxide, cumene hydroperoxide, or 2,5-dihydroperoxy-2,5-dimethylhexane.
 19. The process of claim 18, wherein said initiating of step (b) is accomplished by introducing sulfur dioxide into said suspension. 